Hardness tester and method for measuring the hardness of metallic materials

ABSTRACT

The hardness of a metal sample is determined using a hardness tester provided with an indenter having an indenting tip and means for loading the indenter. The method comprises employing an electrically conductive indenting tip and connecting the indenter and the metal sample to the input terminals of an electric resistance measuring apparatus. The indenter is loaded up to a preset load on a reference metal sample having a known hardness and detecting the contact resistance under the load. The tester is functionally set on a metal sample to be tested and gradually the indenter is loaded until the same electrical contact resistance as detected on the reference sample is measured by the electrical resistance measuring apparatus and the load which is actually applied to the indenter is read. The hardness of the tested sample is determined through a proportionality relationship between the preset load applied to the reference sample and the load applied to the tested sample.

TECHNICAL FIELD

The present invention relates to a hardness tester for metals and metalalloys capable of expressing comparative hardness values in Vikers orBrinnel scales, and providing for a direct reading without the need forcarrying out microscopic indentation size measurement or detectingindenter's displacement.

BACKGROUND ART

Hardness testers are instruments which are well known to the skilledtechnician. The apparatus, which may take different forms, comprises anindenter constituted essentially by a stylus assembly which may assumevarious forms depending on the type of sample to be tested provided withan indenting tip of a hard and undeformable material, commonly a diamondor a hard metal, and a loading system for applying a certain load. Thehardness assessment is generally a multistep test whereby the size ofthe indentation produced by the indenting tip on the surface of thepiece to be tested under a certain load applied to the indenter, or thedepth of penetration of the indenter is measured.

The tip of the indenter may have a conical, a spherical (Brinnel) orsquare piramidal (Vikers) form. The diamond or the hard metal are thematerials commonly used for making these tips which must be sufficientlyhard so as not to be scratched by the material being tested andundeformable under the test load.

A bench-type apparatus comprises usually a support structure, a loadingsystem through which it is possible to load and by an inventory ofimplements for permitting to select a certain configuration of theindenter, suitable for the piece to be tested. e.g. for effecting testson external or internal surfaces of a machined piece of machinery.

Where the indentation cannot be viewed through an optical microscope,for example in the case of an internal surface of a tubular piece, theassessment of the hardness must necessarily be based upon a micrometricdetermination of the depth of penetration of the indenter tip in thematerial being tested, according to the capabilities of the knownapparatuses. These measurements of size and/or depth of an indentationare burdensome and are often a source of evaluation errors.

In U.S. Pat. No. 4,848,141 and in the Article "Measurements of Hardnessat Indentation Depths as Low as 20 Nanometers"; by W. C. Oliver, R.Hutchings and J. B. Pethica, pp 90-108, ASTM Special TechnicalPublication No. 889, 1986, a method of determining hardness, yieldstrength, and other mechanical parameters of a material are disclosedwherein the load applied to a microindenter and the displacement of theindenter after the establishment of contact with the surface of thesample are electronically measured and elaborated to obtain the value ofthe particular mechanical parameter.

On the other hand, the contact area between two bodies is difficult todetermine when the area of contact is less than a few square microns.There have been prior attempts to obtain such a measurement bydetermining the electrical resistance at the junction. However, to date,such a technique has never found application in indenter-type hardnesstesting apparatus which for other reasons must utilize an intrinsicallyinsulating indenter tip, such as a diamond tip.

SUMMARY OF THE INVENTION

There is a necessity of an apparatus for measuring the hardness ofsurfaces different to access to, such as the side of the tooth of a gearor internal surfaces which cannot be examined under a microscope andeasy to be used and which positively reduces the probability of areading error by the operator.

This objective is attained by means of the apparatus objects of thepresent invention, which is advantageously capable of providing aprecise indication of the hardness of a sample in an easy manner whilereducing the probability of an error of assessment by the operator.

Basically the hardness tester of the present invention utilizes aninstrumental determination of an electrical contact resistance fordetermining the hardness of a metallic sample. The contact resistancewhich is measured is the electrical contact resistance between the tipof the indenter and the metallic sample being tested. This resistance isa function of the contact surface area, which is in turn a function ofthe indentation produced by the tip in the metallic sample being tested,under the applied load. To this end, the indenter's tip as well as themetallic sample being tested, are connected to a resistance-test circuitand the indenter's tip is made with a substantially conductive materialhaving an electrical conductivity which is relatively low and preferablylower or equal to about a tenth of the electrical conductivity of themetallic sample being tested. This fact greatly increases thesensibility of the measurement system. In other words, a "concentration"of the resistance for the whole electric path of the resistancemeasuring at the indenter's tip, establishing the contact with themetallic sample circuit which inevitably includes a metallic tip holderand a metallic sample support beside the electrical wire connections tothe input terminals of the resistance measuring instruments orresistance measuring circuit, determines a greater usable gradient ofvariation of measured resistance ΔR in function of a given increment ofthe penetration of the indenting tip in the metallic sample beingtested.

The actual value of the hardness of the sample is preferably expressedby the hardness tester through a comparison with a similar electricalcontact resistance determination performed on the surface of a "mastersample" of precisely known hardness. This comparison may be effected bydetermining the difference between the loads which attain the samecontact resistance on the master sample and on the successively testedsample. In this way, by exactly knowing the hardness of the "mastersample", the hardness of the test sample may be readily read from astandard Brinnel or a standard Vikers scale by utilizing the differencebetween the loads attaining the same contact resistance, i.e. the samearea of contact, i.e. the same size of indentation, to read the correcthardness value of the test sample.

The master sample of known hardness which is used for a particularhardness assessment should preferably be a master having a hardnessvalue which is relatively close to the expected hardness of the testsample and a reasonable inventory of master samples of differentprecisely known hardness will readily permit to accomplish such apreferable condition of comparison.

The comparison method of measurement though implying a two step testprocedure practically elimintates all problems which would be associatedotherwise to the periodical trimming of the apparatus response. Inpractice, the comparison method has been found to be effective inbalancing out the effects of innumerable causes of variance of theelectrical contact resistance which is subject to the influence of thedifferent conditions under which the measurement is carried out. In thisconnection, as it will be further described later in the description,carrying out the test with the surface of both the test sample as wellas of the master sample wetted with oil has been found to furtherimprove the repeatability of the contact resistance measurements andconsequent hardness assessments.

The different aspects and advantages of the invention will be moreeasily appreciated through the following description of severalembodiments of the invention and by reference to the attached drawings.

SUMMARY OF THE DRAWINGS

FIG. 1 is a schematic sectional elevation view of a bench-type hardnesstester according to the present invention;

FIG. 2 is a schematic view of a hardness tester of the present inventionschematically showing a resistance testing circuit.

BEST AND VARIOUS MODES FOR CARRYING OUT INVENTION

For ease of description, the same parts or parts of the measuringapparatuses shown which are functionally equivalent among each other areindicated with the same reference number in the figures.

With reference to FIG. 1, the hardness tester depicted therein comprisesa support base 1, a stanchion structure 2 on which a shelf body 3 may beslidibly set and blocked in a fixed position by means of a blocking knob4. A mobile loading body 9 is assembled on the shelf body 3, and issuspended therefrom by means of two steel leaf-springs 5 and 6 which arefixed at one end to the shelf body 3 by means of the screws 7 and 8. Theloading body 9 is fixed to the other end of the two suspendingleaf-springs 5 and 6 by means of the screws 10 and 11. The mobile body 9may be loaded downward by means of the loading screw 12 acting on themobile body 9 through a compression spring 13 which exerts a force onthe mobile body 9 in opposition to the force exerted on the same body 9by the restraining or suspension spring 14, the pre-compression of whichmay be adjusted by means of the stopper 15.

By interposing a boss 18 of an electrical insulating material, forexample of an insulating ceramic, a "U" shaped indenter holder 17,suited for interior surfaces, is attached to the lower end of the loadpin 16 which is fixedly connected to the mobile body 9 by means of theset screw 19.

The indenter's tip 20 is mounted into a recess at the end of the indeterbody 17. The metallic sample to be tested is identified with 21 and thedetermination of the harness may be carried out on the internal surface22 of the piece.

The metallic sample 21 rests on a metal support 23 (anvil) which iselectrically isolated from the base 1 by placing therebetweenelectrically insulating supports 24, e.g. of a nonconductic ceramicmaterial.

Wire connection 25 and 26 to a resistance measuring apparatus aredepicted.

The load which is applicable by means of the handwheel 12 may bedetected by means of strain gages mounted on the block 27 and theterminals of the strain gages are connected though the cable 28 to asuitable measuring instrument, provided with an analog or digitaldisplay (not visible in FIG. 1) of the load which is applied toindenter.

FIG. 2 depicts schematically a basic system for determining the hardnessby means of the apparatus of the invention.

In FIG. 2 the hardness tester is schematically depicted by utilizing thesame number for identifying the components analog to those alreadydescribed in connection with FIG. 1. In this schematic showing of FIG.2, the load P which is applied to the indenter is displayed on the scale29 where an index 30 indicates the value of the load which is applied tothe indenter 17, i.e. to the indentation tip 20, by acting on theloading handwheel (12 in FIG. 1).

As shown, the electrical wire connections 25 and 26 for the "electricalseries" formed by the conducting metal body 17 of the indenter, theindentation tip 20, the test sample 21 and the anvil 23 and which"series" of electrically conducting elements is isolated by theinsulators 24 and 18, are shown coupled to a Wheatstone test-bridge,indicated with W as a whole, which is powered by the battery B throughthe rheostat RO and the switch S, at a voltage which is measured by thevoltmeter V, while the equilibrium condition of the test-bridge isdetermined by means of the galvanometer G.

The illustration of the electrical contact resistance measuringapparatus in the form of a classical Wheatstone bridge test circuit, hasa clear illustrative and nonlimitative purpose, being the Wheatstonebridge the classical test circuit for resistance measurements. Of courseany other instrument capable of measuring electrical resistance withsufficient degree of precision can obviosuly be utilized in the hardnesstester of the invention, whether or not such an instrument incorporatesa Wheatstone-bridge test circuit.

According to the preferred method of assessing hardness of a test sampleby means of the apparatus of the invention, a reference or blank ormaster sample having a precisely known hardness, sufficiently close inorder of magnitude to the presumed hardness of the surface of the sampleto be tested, is placed on the anvil 23. The hardness of the mastersample which is exactly known may be conventionally expressed in termsof specific load in a Vikers and/or Brinnel scale, whereby the hardnessvalue corresponds to the load expressed in kilograms per squaremillimeter of indentation area produced on the surface of the material.A certain reference load is applied to the indenter by reading it on thescale of the dynamometer 29 and such a reference load may be chosen infunction of the type of surface treatment to which the sample to betested has been subjected, i.e. to the order of magnitude of thepresumed hardness of the sample to be tested. This produces a certainindentation on the surface of the master or reference sample.

Under this loading condition, the Wheatstone bridge is balanced byacting on the variable resistance of the bridge, according to well knownresistance measuring procedures, or the resistance is accuratelydetermined by any other means.

The load is released and the reference sample is substituted with thesample to be tested on the anvil 23 of the instrument.

The dynamometer is progressively and gradually loaded until theWheatstone bridge reaches again a balance condition.

When the bridge is returned to the balance condition the loadcorresponding to such a renewed balance condition of the contactresistance measuring bridge is read off the scale 29 of the dynamometer.

In this way, it is established that the resistance of the electricalseries is equal to the resistance attained when testing the referencesample and because the electrical resistance of the relative branch ofthe Wheatstone bridge between the wire connection 25 and 26 issubstantially represented by the contact resistance between theindenter's tip 20 in function of the contact area of the latter with thesample being tested, the equality of resistance is indicative of anequality of indentation, i.e. of contact area between the indenting tip20 and the metallic sample being tested. At this point the load which isread off the scale 29 of the dynamometer is in the same ratio with theload which was applied on the reference sample for establishing such acontact resistance as the respective hardnesses. For example, if on thereference sample was applied a load of 5 kilograms and on thesubsequently tested sample the load for balancing the contact resistancebridge circuit is of 4,5 kilograms, the hardness of the sample beingtested may be correctly assessed as being 10% lower than the knownhardness of the reference sample.

Such a comparison condition for relatively assessing the hardness willbe as precise as less significant is the electrical resistancecontribution of the metallic masses of the parts 17, 21 and 23,electrically connected in series of FIG. 2, as well as of the wireconnections 25 and 26, in respect to the electrical contact resistancebetween the indenter's tip 20 and the metal sample . An electricalconductivity of the material with which the indenter's tip 20 is made,lower than about 10 times the electrical conductivity of the material ofthe piece to be tested 21 or more generally of the electricalconductivity of metals and metallic alloys in general, permits to attaina remarkable precision in the hardness assessment according to thecomparative method described above.

Such an upper limit of electrical conductivity of the indenter's tip,permits a sufficient sensitivity of the measuring system and allows tosatisfactorily use it also for determining the hardness of samples madeof particularly resistive metal alloys or other materials which areintrinsically very resistive such as titanium, tungsten, "cermets" andthe like. Of course, when the instrument must be used for assessing thehardness on highly conductive metallic materials such as steel, copper,aluminum, etc., the indenter's tip may have a much greater electricalconductivity in absolute terms.

The lower limit of an electrical conductivity of the material with whichthe indenter's tip is made, will be such as to permit the measurement ofan electrical resistance which in absolute terms is not eccessivelyhigh, which fact would otherwise represent a factor of reduced precisionof the resistance measurements because of intrinsic sensitivitylimitations of the electrical resistance measuring instrument.

Materials which may be used for manufacturing a suitably conductiveindenter's tip of the hardness tester may be of various kind althoughpossessing sufficient hardness and undeformability properties under loadand an electrical conductivity fitting the above-mentioned limits.Borides, carbides, nitrides, hydrides, oxyborides, oxynitrides,oxycarbides, perowskites, delofossites, spinels and mixtures of the samematerials may suitably form the indenter's tip of the invention. Thesematerials may be shaped by sintering and diamond ground for producingthe desired profile of the tip. Most preferably the tip is a crystalhaving sufficient electrical conductivity, such as for example a typeIIb diamond crystal. More preferably the electrical conductivity of anindenter's diamond tip is raised by means of an ion implantationtreatment. It has been found that an ion implantation of diamond withnitrogen at a level of 10¹⁶ ions per centimeter square, produces anadequately conducting "skin" of the tip. The implantation is normallycarried out on the diamond tip after it has been already set in theindenter's body and ground to the desired shape, by exposing theindenter assembly to the flux of accelerated nitrogen ions inside an ionimplantation vessel. Of course ions other than nitrogen may besatisfactorily implanted in a diamond crystal for imparting a sufficientelectrical conductivity to at least a superficial "skin" layer of thetip.

When employing an implanted crystal tip set in a metallic indenter body,the electrical continuity between the tip and the metallic indeter'sbody in which the tip is set, may be ensured by depositing a conductingmetal coating, for example by a chemical vapor deposition (CVD) process,after having masked the vertex of the indenting implanted crystal tip.The conductive coating deposited on the unmasked portion of theimplanted crystal tip and on the adjacent surface of the metallicindenter body in which the tip is solidly set, provides a conductive"bridge" between the relatively conductive implanted "skin" portion ofthe crystal tip and the metallic indenter body. This, beside preventinga criticity of electrical continuity between the crystal tip and themetallic indenter body, further "concentrates the resistance signal" inthe contact area established by the indenter's tip with the metallictest sample. Of course the metallic coating on the surface of theindenter's tip is arrested (by the masking) at a sufficient distancefrom the vertex of the indenter's tip so as not to come in contact withthe metallic sample.

According to a preferred method of the invention, the repeatedelectrical resistance measurements (on the reference and on the testsamples) are performed after having wetted the surface of the metalsample with oil, such as a lubricating oil. To this effect the samplesmay also be completely immersed in a pool of oil while measuring thecomparative contact resistances. It has been found surprisingly that byconducting the measurements in oil, the repeatability of the contactresistance measurements under identical load conditions, is enhanced.This may be explained by assuming that the oil dissolves eventualresidual traces of grease which would otherwise remain undisplaced underthe relatively high specific loads at the indenter's tip-sampleinterface; or by a cleansing action of the oil on microscopic metallicparticles whose presence could affect the contact resistance and whichmay be detached and electrically neutralized (isolated) by the oil; orthe action of the oil may be that of increasing the "definition" of thecontact areas in presence of the plastic flow of the indented metal. Analternative procedure to the wetting the surface with oil prior toindent the surface and measure the electrical resistance is toaccurately clean the surface of the sample to be indented. However thislatter technique has been found inferior to conducting the test inpresence of oil, in terms of repeatability of the electrical resistancemeasurements.

Naturally the hardness tester of the invention may be realized indifferent forms adapted to specific uses. The apparatus may beconstructed in the form of a bench-type tester or it may be made in aform of a portable instrument for field use. In this latter form theinstrument may be readily reduced in size and have a simplifiedconfiguration wherein the load can be manually applied by a simplepushing-action. It has in fact been found that the measuring system welltolerates slight oscillations of the tip's axis of application of theload which may occur during the electrical resistance measurements. Thismay be explained by the fact that the electrical contact surface betweenthe indenter's tip and the sample depends basically from the specificload (i.e. from the hardness of the sample). For this reason even forsmall variations of the angle of incidence of the axis of the indeter'stip and the surface being indented, the variations of the contactresistance are negligeable.

Naturally the hardness tester of the invention may be easily automatizedby providing for a motorized gradual load application which may beeasily controlled. For example when repeating the contact resistancetest on a first sample, the load application may be automaticallyarrested upon attainment of equilibrium of the resistance measuringbridge circuit. The automation may also provide for calculation anddisplay of the actual hardness of the tested material. Moreover thesystem may be equipped with a programmable data-recording and processingunit for a complete analysis of the hardness determination.

The electrical contact resistance measurements which are used forcomparing and determining hardnesses, instead of being end-point typedeterminations, may be carried out in the form multiple pointdeterminations under progressively increased loads both on the referencesample of known hardness and subsequently on the sample to be tested.The various pair of comparable contact resistance measurements may beanalysed for providing information on the variation of hardness infunction of the depth of penetration of the indeter in the sample beingtested. Also in a case such as the above, the provision of automaticdata-collecting and data-processing units will greatly simplify and makea thorough analysis of the properties of the tested sample practicallyimmediate.

I claim:
 1. A method for determining the hardness of a metal sample byusing a hardness tester provided with an indenter having an indentingtip of a hard and undeformable material and means for loading saidindenter with a certain load for producing the penetration of said tipin the metallic sample, the depth of penetration and/or the width of theindentation produced under a given load providing a measure of thehardness of the material, characterized by comprising:utilizing anelectrically conductive indenting tip and connecting said indenter andsaid metallic sample to the input terminals of an electric resistancemeasuring apparatus; loading said indenter up to a preset load on areference metallic sample having a known hardness and detecting thecontact resistance under said load; functionally setting the tester on ametallic sample to be tested and gradually loading the indenter untilthe same electrical contact resistance as detected on said referencesample is measured by said electrical resistance measuring apparatus andreading the load which is actually applied to the indenter; determiningthe hardness of the tested sample through a proportionality relationshipbetween said preset load applied to the reference sample and the loadapplied to the tested sample for attaining the same electrical contactresistance on the reference sample and on the test sample.
 2. The methodas defined in claim 1, wherein the testing is conducted with the surfaceof the sample being indented wetted with an oil.
 3. The method asdefined in claim 1 wherein said indenting tip has an electricalconductivity which is not greater than 1/10 of the electricalconductivity of said reference metal sample and/or of said metal sampleto be tested.
 4. The method as defined in claim 1, furthercomprisingrepeatedly recording paired values of applied load andcorresponding electrical contact resistance during the loading up of theindenter to a preset load on said reference sample; repeatedly recordingpaired values of applied load and corresponding electrical contactresistance while gradually loading the indenter on the metallic sampleto be tested; determining the hardness of the tested sample through aproportionality relationship between loads corresponding to identicalelectrical contact resistances detected on the reference sample and onthe tested sample, for a certain interval of the applied load.